July 7, 2011 -- Being very thin or fat appears to increase the odds that a man will experience some kind of sexual difficulty, but the same may not be true for women, a new study shows.

The study, a survey of more than 5,500 adults in Denmark, finds some surprising and complicated associations between lifestyle factors and sexual health.

In men and women, for example, body size -- being either too thin or
obese -- appears to be related to the risk of having sexual problems. For men, those extremes increase the risk. Women in those ranges, on the other hand, appear to have less sexual difficulty.

The study shows that being very thin is associated with 22 times the risk that a man will experience some kind of persistent sexual dysfunction, including erectile dysfunction (ED), inability to orgasm, premature ejaculation, and pain with sex, compared to men who are normal weight.

Similarly, men with very large waists, over 40 inches, had a 71% increased risk of having some kind of sexual trouble compared to men who had a waist size under 37 inches.

"What I found interesting was that the number of men who were underweight, or had low BMI [body mass index], had various elevated risks for sexual difficulties," said Edward O. Laumann, professor of sociology and an expert in human sexuality at the University of Chicago. He was not involved in the current research.

He says having a large waist and a high BMI, which is also associated with health conditions like heart disease and diabetes, has long been known to be associated with an increased risk of sexual dysfunction in men.

Understanding why having a low BMI might also increase the risk is "less straightforward to figure out" and will require more research, he says.

Women, Weight, and Sex

Being underweight appeared to cut a woman's chances of having sexual difficulties in half compared to a normal-weight, sexually active woman.

And obese women, with BMIs over 30, had 70% less chance of experiencing some kind of sexual dysfunction than their normal-weight counterparts.

Sexual problems tracked among women in the study including vaginal dryness, inability to orgasm, pain with intercourse, and vaginismus -- involuntary spasms that tighten or close the vagina.

"The differences between men and women with respect to sexual dysfunction were surprising," says study researcher Morten Frisch, DSc, associate professor at the Staten Serum Institute in Copenhagen, Denmark. "There may be a plausible explanation. Women who experience trouble may quit having sex altogether while men may continue, particularly into the older ages."

Among other surprises in the study for women: Those who reported using marijuana in the last year were about three times more likely than those who did not to report being unable to orgasm.

And women were intense exercisers and competitive athletes were more than four times more likely than those who were light exercisers to report experiencing vaginismus.

"People know very well that these lifestyle factors are not good for their health in terms of heart disease and cancer and other things. Nevertheless, there is a continued consumption of these drugs and unhealthy lifestyles and they affect sexual health as well as physical health," says Frisch.

The study is published in the Journal of Sexual Medicine.

Tracking Sexual Health

For the study, researchers scoured data collected by the Danish Health and Morbidity Program, which randomly surveys citizens 16 years of age and older.

Study participants were interviewed in person and again later answered questions privately by questionnaire.

They were asked if they'd been sexually active with a partner within the last year. Those who had were then asked about the presence of sexual difficulties.

The study found that 18% of men and 23% of women had not had sex with a partner in the previous year.

For both men and women, being physically inactive was tied to a higher risk of also being sexually inactive.

Men who reported getting no exercise in their leisure time were 78% more likely than men reporting at least light physical activity to say they had not had sex with a partner in the last year.

Women who were physically inactive were about 45% more likely than light exercisers to report being sexually inactive.

Smoking, Waist Size Affect Frequency of Sex

For women, other things that also increased the risk of being sexually inactive were
smoking, being a teetotaler, and having a body mass index (BMI) over 30 or a waist size over 34 inches.

For men, having a very large waist size, over 40 inches, was also associated with a higher risk of being sexually inactive.

Sexually active women who reported any marijuana use within the past year were nearly three times as likely as those who didn't use the drug to report being unable to orgasm.

Female athletes who were sexually active had nearly four times the odds of experiencing vaginismus.

For men, being underweight or obese, with a body mass index (BMI) under 20 or over 30, increased the odds of having regular trouble with erectile dysfunction (ED).

Men who were underweight were nearly eight times as likely to report frequent premature ejaculation as normal-weight men.

Male current or former smokers were nearly twice as likely as men who had never smoked to experience ED.

Researchers are careful to point out that their findings are only associations. They cannot prove that these lifestyle factors are causing the sexual problems, only that the two are occurring together.

But they say it's likely that improving general health could also improve sexual health.

"Losing weight, getting in better shape, coughing less, being less exhausted when you get back home, it's plausible that you may improve your general health and your sex life," Frisch says. "I think those things go together."